This invention relates to a method of aligning an end of a polarization-preserving optical fiber for attachment to a linearly polarized (plane polarized) semiconductor laser.
The performance of various optical equipment such as electro-optic sensors, fiber gyroscopes, and coherent communications equipment can be considerably enhanced by using optical fibers which preserve the polarization of the light they carry. At least two types of polarization-preserving optical fibers are known in the art. One type has a core which is very elliptical. The other type is made of a birefringent material, i.e., a material which exhibits different indices of refraction for different polarizations. However, polarization in this second type of fiber is maintained by means of two distinct axes of propagation determined by the birefringent dielectric medium of the core material. Either type of polarization-preserving fiber has the property that light entering the fiber which is polarized parallel to the optical axis of the fiber (i.e., the polarization-preserving axis) will keep its polarization while propagating within the fiber. Polarized light entering off-axis will be scattered while propagating through the fiber, that is, it will lose its polariza- tion.
A measure of the degree of polarization of light is the ratio of maximum transmitted intensity, which will occur in the plane of polarization, to the minimum transmitted intensity, which will occur in the plane orthogonal to the plane of polarization. This ratio is called the extinction ratio, or the polarization ratio, and is usually expressed in decibels.
Polarization-preserving fibers such as those described above are capable of maintaining an extinction ratio in excess of 25 dB.
The first step in obtaining well-polarized output from a polarization-preserving fiber is to provide it with sufficiently polarized light at the start. The total light output of most semiconductor lasers, however, is relatively poorly polarized. Typical extinction ratios for such devices, considering total light output over the entire area of the beam, are only on the order of 10-12 dB. In semiconductor lasers where the light mode is guided internally by the gain of the active medium, the emission at the outer edges of the medium is largely dominated by absorption. This results in a beam which has a spatial distribution of unpolarized emission which decreases from the edge of the beam to the center of the beam so that there is better polarized light toward the center of the beam than there is at its edges.
The next step in obtaining well-polarized output is proper alignment of the optical axis of the fiber with the direction of polarization of the light source. As stated above, this maximizes the percentage of light which will stay polarized in the fiber. Due to the poor polarization of the light emitted by a semiconductor laser as described above, an alignment set-up such as that depicted in FIG. 1 has been used. The end of fiber 10 is placed within a suitable jig 30, such as a collet-type jig. The end of fiber 10 protruding from jig 30 has been greatly exaggerated for clarity. Interposed between the receiving end of fiber 10 and laser 20 is an optical system designated generally by 40, which typically comprises a convex lens 50, a polarizer 60, and another convex lens 70. The lenses 50 and 70 are used because light from a semiconductor laser is generally highly divergent, so that lenses are necessary to collimate the beam. The emitting end of fiber 10 emits the transmitted laser light into yet another convex lens 80, from which the light travels to a polarizer 90 and detecting means 100. Use of optical system 40 ensures a light source having an extinction ratio on the order of 50-70 dB. Using this apparatus, the fiber is rotated by rotator 35 and the extinction ratio measured, until a maximum in the extinction ratio indicates that the fiber is aligned, i.e., that most of the light is keeping its polarization and not being scattered in the fiber to produce a large amount of light off-axis.
The apparatus described above is not, however, well-suited to perform an alignment in a process for permanent attachment of the fiber to the laser. This is because prior to attachment, all of the coupling optics have to be removed, and then the fiber has to be moved to the laser over relatively long distances. These steps which intervene between alignment and attachment increase the chances that the fiber will ultimately be attached in a misaligned position.